Custom Xquery code to add tow values

xquery version "1.0" encoding "Cp1252";

declare namespace xf = "http://tempuri.org/Test/Addition/";

declare function xf:Addition($input1 as xs:string,$input2 as xs:string)
    as xs:string {
   
   
    let $add as xs:string := fn:concat($input1," ",$input2)
    return
    xs:string($add)
       
};

declare variable $input1 as xs:string external;
declare variable $input2 as xs:string external;

xf:Addition($input1,$input2)